There are no good ways to die, but death by the West Nile virus is worse than most. The tropical disease begins in birds, which pass it on to mosquitoes that then go on to infect human beings with a bite. Most people who contract West Nile don’t experience any symptoms at all, but about 1 in 5 suffer fever, headaches and body aches, usually lasting a week or so. A far less lucky 1 in 150 experience high fever, tremors, convulsions, paralysis and coma. Some — especially the immunocompromised and the elderly — die.

That’s what makes the major outbreaks of West Nile virus in the U.S. this summer so scary. So far, nearly 700 cases have been reported to the Centers for Disease Control (CDC), including 26 deaths — the biggest nationwide outbreak since 2004. The situation is particularly bad in and around Dallas, where the virus has killed 10 people and sickened more than 200 so far this summer. The city has declared a state of emergency and, for the first time in 45 years, has begun aerial spraying of pesticide to kill the mosquitoes that are the virus’ delivery system — even in the face of residents’ heated concerns that the pesticide could be more dangerous than the disease. “I cannot have any more deaths on my conscience because we failed to take action,” Dallas Mayor Mike Rawlings told reporters recently.

Why has the summer of 2012 proved so hospitable to the West Nile virus and the mosquitoes that carry it? Like so much else that’s gone this season, blame the weather. An extremely mild winter throughout much of the country allowed more mosquitoes than usual to survive, while the unusually high temperatures this scorching summer further increased their numbers as well as speeding up their life cycle, causing more of the virus to build up in their salivary glands. (West Nile—which originated in Uganda—was first discovered in the U.S. during the very hot summer of 1999, in New York City.) Dallas in particular, ground zero of the outbreaks this year, had a rainy spring, which left more standing water — ideal nurseries for mosquito eggs. The housing crisis may play a role in the spread as well: many foreclosed homeowners have abandoned their properties, sometimes leaving behind swimming pools that make excellent mosquito breeding grounds.

Ultimately, officials in Texas and other hard-hit states should be able to bring the outbreak under control, both through pesticides and by draining pools of standing water. As the summer ends, so will West Nile season. But thanks to climate change, this isn’t likely to be the last time a disease we’d usually associate with the tropics makes its way into the U.S. The mosquito-borne dengue fever, which is endemic in much of the tropics, has been reported in south Texas, as well as the Florida Keys. The first U.S. case of Chagas disease, a deadly Latin American infection transmitted by a cockroach-like bug that can feed on human blood, was reported last month. As the climate continues to warm worldwide, the zone of risk for all these diseases and even malaria could continue to expand.

But things aren’t as simple as just “hotter temperatures equals more disease.” That’s because there’s another factor at work: us — or, more specifically, our policymakers. The severity of tropical diseases is also a matter of whether or not governments are capable — and willing — to defend their populations against infections. In a 2010 paper in Nature, researchers noted that even though global temperatures increased throughout the 20th century, the range of malaria actually contracted, as countries developed economically and put in place measures to control the disease. Malaria was once endemic in southern U.S. states like Florida and Georgia until after World War II, when it was eradicated thanks to the work of the CDC, which was created in 1946 with the stated goal of controlling the disease. Today tropical infections like malaria and dengue are as much diseases of poverty as they are of climate, which is why nearly 200,000 people die from malaria each year in the desperately poor Republic of Congo, while the disease has been eliminated in the tropical but rich island nation of Singapore. It isn’t being hot alone that kills — it’s being poor.

That’s one more reason southern states like Texas will be the first to feel the push of new tropical diseases, as Peter J. Hotez of the National School of Tropical Medicine pointed out in a recent piece for the New York Times. The South is warmer than the rest of the country, of course, and with climate change it’ll keep getting warmer. But the South is also the poorest region in the U.S. — especially in Gulf Coast states like Louisiana and Alabama, where the poverty rate can push 20%. If we can’t fix that problem, this summer’s brush with West Nile will look like a bug bite compared to the troubles to come.